Quadratic formula

Here you will learn about the quadratic formula and how you can use it to solve quadratic equations.

Students will first learn about the quadratic formula as part of algebra in high school.

What is the quadratic formula?

The quadratic formula is used to solve quadratic equations by finding the roots, x.

The quadratic formula is: x=\cfrac{-b\pm\sqrt{b^2-4ac}}{2a}

By using the general form of a quadratic equation, a x^{2}+b x+c=0, you can substitute the values of a, b and c into the quadratic formula to calculate x (the solution(s) for the quadratic formula).

The \pm in front of the square root means ‘plus or minus.’ This accounts for the positive root and negative root.

Quadratic equations normally have two solutions, so you need to use the formula twice, once with a + and once with a –.

x=\cfrac{-b+\sqrt{b^2-4ac}}{2a}\quad x=\cfrac{-b-\sqrt{b^2-4ac}}{2a}

To find out how many real roots a quadratic equation has, use the expression under the square root in the quadratic formula → b^2-4 a c. This is called the discriminant.

• If b^{2}-4ac > 0, this means there are two solutions (these are real numbers).

• If b^{2}-4ac=0 , this means there is one solution (which is a real number).

• If b^{2}-4ac < 0, this means there are no real solutions, here the solutions will be imaginary numbers.

For example,

Let’s see how many real solutions this quadratic equation has.

Because the discriminant is greater than 0, there are two solutions (real numbers).

Now, let’s solve for the two solutions.

\begin{aligned} & x=\cfrac{-3+\sqrt{3^2-4 \times 1 \times 2}}{2 \times 1} \\\\ & x=\cfrac{-3+\sqrt{1}}{2 \times 1} \\\\ & x=\cfrac{-3+1}{2} \\\\ & x=\cfrac{-2}{2} \\\\ & x=-1 \end{aligned} \quad \begin{aligned} & x=\cfrac{-3-\sqrt{3^2-4 \times 1 \times 2}}{2 \times 1} \\\\ & x=\cfrac{-3-\sqrt{1}}{2 \times 1} \\\\ & x=\cfrac{-3-1}{2} \\\\ & x=\cfrac{-4}{2} \\\\ & x=-2 \end{aligned}

You can check by graphing the solution. Remember, the solutions are the x -intercepts.

What is the quadratic formula?

Common Core State Standards

How does this relate to high school math?

• Algebra – Reasoning with Equations and Inequalities (HSA.REI.B.4b)
  Solve quadratic equations in one variable. Solve quadratic equations by inspection ( e.g., for x^2=49), taking square roots, completing the square, the quadratic formula and factoring, as appropriate to the initial form of the equation.
  
  Recognize when the quadratic formula gives complex solutions and write them as a \pm bi for real numbers a and b.

How to use the quadratic formula

In order to use the quadratic formula:

1. Identify the value of \textbf{a, b} and \textbf{c} in a quadratic equation.
2. Substitute these values into the quadratic formula.
3. Solve the equation with a \bf{+}, and then with a \bf{-}.

Quadratic formula examples

Example 1: equation of the form ax² + bx + c = 0 where a = 1

Solve x^{2}-8x+15=0.

1. Identify the value of \textbf{a, b} and \textbf{c} in a quadratic equation.

2Substitute these values into the quadratic formula.

Using parentheses will help to make the calculation clear.

3Solve the equation with a \bf{+}, and then with a \bf{-}.

\begin{aligned} & x=\cfrac{-(-8)+\sqrt{(-8)^2-4(1)(15)}}{2(1)} \\\\ & x=\cfrac{8+\sqrt{64-60}}{2} \\\\ & x=\cfrac{8+2}{2} \\\\ & x=5 \end{aligned}\quad \begin{aligned} & x=\cfrac{-(-8)-\sqrt{(-8)^2-4(1)(15)}}{2(1)} \\\\ & x=\cfrac{8-\sqrt{64-60}}{2} \\\\ & x=\cfrac{8-2}{2} \\\\ & x=3\end{aligned}

When you plot the graph of the quadratic equation you get a special ‘U’ shaped curve called a parabola.

By using graphing techniques such as this, you can see that the roots of the quadratic formula are where the quadratic graph crosses the \textbf{x} -axis or the x -intercepts.

You also can check that the solutions are correct by substituting them back into the quadratic equation.

Example 2: solutions of the quadratic equation with a > 1

Solve 2x^{2}+5x+3=0.

Solve the equation with a \bf {+}, and then with a \bf {-}.

\begin{aligned} & x=\cfrac{-(5)+\sqrt{(5)^2-4(2)(3)}}{2(2)} \\\\ & x=\cfrac{-5+\sqrt{25-24}}{4} \\\\ & x=\cfrac{-5+1}{4} \\\\ & x=-1 \end{aligned}\quad \begin{aligned} & x=\cfrac{-(5)-\sqrt{(5)^2-4(2)(3)}}{2(2)} \\\\ & x=\cfrac{-5-\sqrt{25-24}}{4} \\\\ & x=\cfrac{-5-1}{4} \\\\ & x=-1.5\end{aligned}

You can see the roots of the quadratic equation are where the quadratic graph crosses the x -axis.

You can also check that the solutions are correct by substituting them back into the quadratic equation.

Example 3: solutions of the quadratic equation with a < 1

Solve -6x^2+x+2=0.

Solve the equation with a \bf {+}, and then with a \bf {-}.

\begin{aligned} & x=\cfrac{-(1)+\sqrt{(1)^2-4(-6)(2)}}{2(-6)} \\\\ & x=\cfrac{-1+\sqrt{1-(-48)}}{-12} \\\\ & x=\cfrac{-1+7}{-12} \\\\ & x=-\cfrac{1}{2} \end{aligned} \quad \begin{aligned} & x=\cfrac{-(1)-\sqrt{(1)^2-4(-6)(2)}}{2(-6)} \\\\ & x=\cfrac{-1-\sqrt{1-(-48)}}{-12} \\\\ & x=\cfrac{-1-7}{-12} \\\\ & x=\cfrac{2}{3}\end{aligned}

You can see the roots of the quadratic equation are where the quadratic graph crosses the x -axis.

You can also check that the solutions are correct by substituting them back into the quadratic equation.

Example 4: simplify the given equation to main quadratic form

Solve x^2+14=9 x.

Solve the equation with a \bf {+}, and then with a \bf {-}.

\begin{aligned} & x=\cfrac{-\left(-9)+\sqrt{(-9)^2-4(1)(14)}\right.}{2(1)} \\\\ & x=\cfrac{9+\sqrt{81-56}}{2} \\\\ & x=\cfrac{9+5}{2} \\\\ & x=7 \end{aligned} \quad \begin{aligned} & x=\cfrac{-(-9)-\sqrt{(-9)^2-4(1)(14)}}{2(1)} \\\\ & x=\cfrac{9-\sqrt{81-56}}{2} \\\\ & x=\cfrac{9-5}{2} \\\\ & x=2\end{aligned}

You can see the roots of the quadratic equation are where the quadratic graph crosses the x -axis.

You can also check that the solutions are correct by substituting them back into the quadratic equation.

Example 5: quadratic equation with complex roots

Solve x^2-x+1=0.

Solve the equation with a \bf {+}, and then with a \bf {-}.

\begin{aligned} & x=\cfrac{-(-1)+\sqrt{(-1)^2-4(1)(1)}}{2(1)} \\\\ & x=\cfrac{1+\sqrt{1-4}}{2} \\\\ & x=\cfrac{1+\sqrt{3} i}{2} \\\\ & x=0.5+0.866 i \end{aligned} \quad \begin{aligned} & x=\cfrac{-(-1)-\sqrt{(-1)^2-4(1)(1)}}{2(1)} \\\\ & x=\cfrac{1-\sqrt{1-4}}{2} \\\\ & x=\cfrac{1-\sqrt{3} i}{2} \\\\ & x=0.5-0.866 i \end{aligned}

Note, since \sqrt{1-4}=\sqrt{-3}, you use a complex number to represent the square root.

The roots are imaginary numbers, because this parabola does not cross the x -axis.

Example 6: quadratic equation with complex roots

Solve 2x^2-7 x+19=0.

Solve the equation with a \bf{+}, and then with a \bf{-}.

\begin{aligned} & x=\cfrac{-(-7)+\sqrt{(-7)^2-4(2)(19)}}{2(2)} \\\\ & x=\cfrac{7+\sqrt{49-152}}{4} \\\\\ & x=\cfrac{7+\sqrt{103 i}}{4} \\\\ & x=1.75+2.537 i \end{aligned} \quad \begin{aligned} & x=\cfrac{-(-7)-\sqrt{(-7)^2-4(2)(19)}}{2(2)} \\\\ & x=\cfrac{7-\sqrt{49-152}}{4} \\\\ & x=\cfrac{7-\sqrt{103 i}}{4} \\\\ & x=1.75-2.537 i \end{aligned}

Note, since \sqrt{49-152}=\sqrt{-103}, you use a complex number to represent the square root.

The roots are imaginary numbers, because this parabola does not cross the x -axis.

Teaching tips for the quadratic formula

• Let students explore other strategies for finding roots before introducing the quadratic formula, such as graphing or factoring the quadratic equation.

• Once students start using the quadratic formula, have them explain the meaning of their answers. This ensures that they do not forget what they are solving for. Better yet, be sure to incorporate many real world examples of the quadratic equation and require students to explain the roots within the context of the problem.

Easy mistakes to make

• Forgetting to complete both calculations of the \textbf{x} value
  When writing the quadratic formula, always write it with the plus and minus symbol. The quadratic formula needs to be solved twice, once with the minus sign and once with the plus sign. This is because there is a positive and negative solution for the square root of a number.

• Not including the sign in front of \textbf{a, b} and \textbf{c}
  When writing the coefficients of the variables for the values of a, b and c, you need to include the sign in front but not the variable.
  For example,
  

  x^{2}-8x+15=0

  so here b=-8, NOT 8 or -8x

• Typing incorrectly into a calculator
  Mistakes can be made when typing into a calculator.
  For example, for
  

  2x^{2}+5x+3=0,
  a=2,\qquad b=5,\qquad c=3

  Use the fraction button and brackets to carefully type into the calculator
  
  

  

  
  Use the arrow buttons to change the + on the top left of the calculation to a - to calculate the second solution.
  
  

  

• Forgetting that sometimes there are no real roots to a quadratic equation or quadratic function
  For example,
  

  x^{2}-4 x+5=0

  
  

  

  You can see that there are no values of x that give a y value of 0.
  The graph does not cross the x -axis. The use of the quadratic formula will result in roots that are not real.

Related quadratic equation lessons

• Solving quadratic equation
• Solving quadratic equations by graphing
• How to factor quadratic equations

The next lessons are

• Radicals
• Algebraic fractions
• Vectors

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